Phase shifters are widely used in a variety of wireless communications. For example, phase shifters are used in antenna arrays, circularly polarized antennas, image rejecting mixers, QPSK receivers, etc. There are lots of applications, especially for variable phase shifters, which have tunable insertion phase. The variable phase shifter provides flexibility when signals with different phases are required. By using a variable phase shifter, an antenna array can be configured to be a phased array, which provides beam steering ability. When a variable phase shifter is used to provide the phase of quadrature signals for driving a circularly polarized antenna, the phase can be offset at different frequencies to widen the axial ratio bandwidth. This phase offset technique can be applied to the image rejecting mixer and QPSK receiver too.
However, a difficulty exists because phase shifters may deviate from their specified or expected performance. Sometimes, the phase shift injected by a phase shifter, especially a variable phase shifter, may change according to the surrounding environment or vary over time. This error in the signal phase may introduce a beam steering error into a phased array, resulting in a reduction in maximum gain. An error in signal phase may greatly degrade the image rejection ability of an image rejecting mixer. Indeed, phase error may reduce the performance of many different applications.